Heated cathode electron multiplier



May 26, 1953 s. NEVIN HEATED CATHODE ELECTRON MULTIPLIER Filed Jupe 19,A 1950 Oom ...zink

INVENToR scarf Nev/N ATTORNEY unOJNZLm Patented May 26, 1953 RATED CATHODE ELECTRGN DIUITIPII'IER' 'Scott 'Nerim Fort Wayne, Indiassignor ftoFannsf-,

-worth Research C'm'poration';V Fort Waynegzlnd.,

a `corporation of Indiana Application June 19, 1950,'S`e1ia1T.' 169,033

(Cl. .Sss-103) i 6v Claims.

This inventionrelates to electron multiplier tubes; and"more'pariicuiarly to tubes ofthe type wherein electronsl are` emitted from a heated oxide coatedprimaryf emitter and strike a secondary emissive target.l

In electron multipliertubes'oi the type wherein secondary emission is eiectedby electrons from aI cathode within the tube, it is desirable to Ause indirectly'heatedcathodes of the oxide coated type toob'tain highinitial emission in the interest of emciency; Considerable trouble has been encountered insuch tubes, in that while initially high-'secondary to primary emission ratios may bel obtained, thisratio rapidly vdeclines in use. It has been thought that poisoning of thesecondary emissive target electrode `is caused bytransfer of substances from the primary cathode oxide coating tol the secondary'emissive layer. Consequently attempts to" overcome this deflciency have been directed to a .study of different oiidecoatin'gsmr theI cathode, and tol structural designs calculated to avoid direct transferof substances betweenl the'emitting electrodes, by evaporationorthe like. Applicant'has discovered that the `diiilcultie'sdescribed yabove can be overcome without resort to the expedients of special tube designs or cathode coating oxides. This has `been accomplished by substituting tantalum as the supporting basemetal ior the emitting cathode-oxide in place oi the usual nickel base.

It is'an object of thisinvention `to provide `an electron multiplier tube giving a' highratio of primary to secondary emission,V stable over a long life period by providing a .cathode comprising ta tantalurn element coated wi-th an emissive oxide.

In accordance with a feature of this invention there is provided an electron multiplier tube comprising a primary emissive cathode, a seconclary emissive electrode and a collector electrode, in which the cathode comprises a heated tantalum base coated with an electron emissive oxide.

The secondary emissive, or target, electrode may comprise known types of elements having a normally high secondary to primary electron emission ratio as for example a magnesiumsilver alloy, which may be treated by an oxidizing process.

The collector electrode may consist of a wire grid, preferably a refractory metal such as tantalum. The oxide coating of the cathode may be of any known variety as for example an oxide of barium-strontium carbonate. Various forms of signal control 'fmayf-be used -suchkasa control.

grid; or'fother" type of sig-nai control may be supplied.s The 'tube maybeconstructed with the various"- electrodea' concentric and coaxial one with another.

'lhe'fabove-mentioned .an'dfother-y features and objects l oivthis, invention zand vthe manner of attai-ning"theml will-becomemcre apparent and the invention'itself willbebest understood by reference -to' the'y following-'description of an embbd-i-ment of fthefinvention itaken inl conjunction with the accompany/inadrawings,- in which:

1 illustrates al tube embodying'theleatures of this invention;-

Qfisfaviewl'oi 'a cathode embodied in my inventionpartlyfbrokenf away showing; the fea tures of construction', and

Fig; 3 'isa setfof E curves 'illustrating typical lif e test*measurernents"onl tubes in accordance with the invention;`

Turning to Fig. l 'thereis disclosed an electron multiplier tube'having'anenvelope i, a cathode tja secondary emitting, ontarget, electrode 3, a control grid". 4 and a collector electrode" 5.

' Heater leads for the cathodeV are shown at t, and

other. leads, for applyingoperating potentials are shown at 1,'8, Band l 0 `for the cathode, secondary emitte1- and. collector electrode respectively. Supportingdiscs ofimica or other suitable ma-s terial shown `at yl I The'rcathode `icomprises.,a base metal, .shown here` as a .sleeve l2carryirugr anemitting coating The'coating.n1a.y. be any, well known type ofxoxi'de; for. example anoxide of barium strontium carbonate.

The secondary emissive electrode Amay for ex" ample be a silver-magnesium alloy oxidized properly to be highly emissive with respect to primary electrons impinging thereon. The collector as shown is in the form of an open cage or grid of wires which will permit the passage of high velocity electrons from the cathode to the secondary emitting electrode.

Normally the secondary emitting, or target, electrode is maintained positive with respect to the cathode, and the collector is maintained at at even higher positive potential. Primary electrons emitted from cathode 2 are accelerated by the positive potential on collector electrode 5 so that most of them pass through the openings in this electrode and impinge on the target electrode 3, releasing secondary electrons. These released electrons tend to migrate to collector electrode 5 from whence they are applied over 3 the lead conductor 9 to a suitable output apparatus.

It is desirable to obtain a high secondary to primary electron ratio K, in the interest of eciency. In the past with this type of tube it has been found that in many instances while K may have a fairly high value, such as 3 or more initially, this value tends to drop upon operation, and may become quite low, for example, 1.5 or so as the tube is used. In tubes of this type it has generally been the practice to use nickel as the cathode base, because it has properties which are considered desirable for indirectly heated cathodes. In making tubes with such nickel supporting sleeves it has been observed that the Secondary to primary emission ratio seems generally to deteriorate rapidly in use. This indicates that in operation some deposit is made on the target electrode which poisons the surface reducing its secondary emissivity. A typical performance curve of a tube using a nickel supporting sleeve is shown at I4, in Fig. 3. It will be observed that while the K ratio is nearly 3 initially, it rapidly dropped off to the neighborhood of 2 and remained there for 350 hours of life test.

In tubes in accordance with this invention, the cathode base support is made of tantalum instead of nickel. In tubes made with such tantalum supporting sleeves, it has been observed that the K ratio builds up to an optimum value of 3 or higher and is maintained at this high value for the period of long life tests. Two typical examples of life test curves are shown at l5 and I6, Fig. 3. The tube of curve I5 started with an initial K ratio of 2.5, built up rapidly to a value of 3.25. and after 649 hours of operation still had a K ratio of 3.1. The tube of curve I6 started at a K ratio of about 3.1 built up to a ratio of 4 after 75 hours operation, and was still operating at a K ratio of 3.5 after 600 hours of operation.

Experiments were made only with tantalum in place of the nickel supports. The precise reason for the improved operation observed is not known but it is believed it may be explained on the basis of vapor pressure of the base metal. At an operating temperature of 1100 C. nickel has a vapor pressure of 15 11o mm..of mercury. In order to obtana comparable vapor pressure tantalum must be heated to over 2400 C. It seems therefore that the nickel vapor may be the contaminant which poisons 'the secondary emitting surface. On this basis other metals of low vapor pressure might be used in placevof the tantalum with comparable results. Thus, one might use tungsten or molybdenum as such supporting sleeves for the emitting oxide of the primary cathode.

Although a grid controlled multiplier has been illustrated by way of example, it should be clearly understood that the principles of the invention apply equally well to tubes wherein signal control is by other means than a grid, as by beam deflection, for example.

While I have described above the principles of my invention in connection with specic apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

What is claimed is:

1. An electron tube comprising, an electron tube envelope, a secondary electron emissive target electrode, a heated type primary electron emissive cathode electrode, and a collector electrode positioned within said envelope, said cathode electrode comprising a tantalum base coated with an emissive oxide.

2. An electron discharge tube according to claim 1 wherein said secondary electron emissive target electrode comprises a surface of silvermagnesium alloy and oxides of these metals.

3. An electron tube according to claim 2 wherein said emitting oxide comprises an oxide of barium-strontium carbonate.

4. An electron multiplier tube in which the ratio of secondary emission to primary emission is maintained stable at an optimum value under operating conditions for a long life period of use of the tube, comprising a hollow cylindrical secondary emissive target electrode, an indirectly heated type primary emissive cathode comprising a tantalum sleeve coated with an emitting oxide positioned coaxially of said target electrode and a collector electrode in the form of a wire cage of tantalum mounted between said cathode and said target electrode.

5. An electron multiplier tube according to claim 4 further comprises a control grid electrode maintained between said cathode and said target electrode.

6. An electron tube according to claim 6 wherein said emissive target electrode comprises an interior surface of magnesium silver alloy oxide.

SCO'I'I NEVIN.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,205,500 Strutt et al June 25, 1940 2,393,803 Nelson Jan. 29, 1946 2,411,601 Spencer Nov. 26, 1946 2,430,218 Eitel et al Nov. 4, 1947 2,462,869 Kather Mar. 1, 1949 

1. AN ELECTRON TUBE COMPRISING, AN ELECTRON TUBE ENVELOPE, A SECONDARY ELECTRON EMISSIVE TARGET ELECTRODE, A HEATED TYPE PRIMARY ELECTRON EMISSIVE CATHODE ELECTRODE, AND A COLLECTOR ELECTRODE POSITIONED WITHIN SAID ENVOLOPE, SAID CATHODE ELECTRODE COMPRISING A TANTALUM BASE COATED WITH AN EMISSIVE OXIDE. 